1. Field of the Invention
This invention concerns a phosphate ion selective electrode, which enables selective measurement of phosphate ion concentration, which is regarded as an important indicator of eutrophication in lakes, marshes, etc., and a method of manufacturing this phosphate ion selective electrode.
2. Description of the Prior Art
An ion selective electrode (also referred to as xe2x80x9cion electrodexe2x80x9d) responds selectively to an ion for which the electrode is designed to analyze and is used to measure ion concentrations, ion activities, etc.
Such an ion electrode responds to a specific ion and indicates a potential difference across a reference electrode in accordance with the ion concentration, thereby enabling the quantification of the targeted ion. For example, with a negative ion selective electrode, the Nernst equation holds between the activity ax of the measured ion (Xnxe2x88x92) and the potential difference E (mV) indicated by the negative ion selective electrode, and at 25xc2x0 C., the following proportional relationship holds between the potential difference E and the logarithm of the activity ax:   E  =                    E        0            ⁢              xe2x80x83            ⁢              (                  reference          ⁢                      xe2x80x83                    ⁢          potential          ⁢                      xe2x80x83                    ⁢          difference          ⁢                      xe2x80x83                    ⁢                      (            mV            )                    ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          system                )              +                  59.16        n            ⁢              xe2x80x83            ⁢      log      ⁢              xe2x80x83            ⁢              a        x            
The activity ax of the targeted ion can thus be calculated simply from the measured value of the potential difference E.
As a prior-art ion electrode that responds to phosphate ions, there is the liquid membrane electrode, in which an ionophore is impregnated and fixed in a polymer substance. However, this type of electrode generally has a high detection limit and is poor in stability.
On the other hand, a solid membrane electrode, with which the sensing membrane is comprised of a solid membrane having the slightly soluble phosphate salt of zinc phosphate as the main component, has also been proposed (refer to Japanese Unexamined Patent Publication No. 130262 of 1992). However the detection limit of this type of electrode is too high for measurement of phosphate ion concentrations down to low concentrations.
In order to achieve the above object, this invention provides a phosphate ion selective electrode comprising a sensing membrane made of a solid membrane having a slightly soluble metal salt as the main component.
Another mode of this invention provides a phosphate ion selective electrode manufacturing method characterized in that the slightly soluble metal salt of aluminum phosphate is used as the main component, the powder of this aluminum phosphate is subject to hot press forming, and the solid membrane that is obtained is used as the sensing membrane.
This invention provides the effect that a phosphate ion selective electrode, which is lower in detection limit and enables measurements down to lower concentrations in comparison to prior-art sensing membranes, can be provided.